This invention relates to circuits and devices for protecting radio frequency ("RF") instruments from accidental injection of high power RF signals. In particular, the present invention is a reverse power protection ("RPP") limiter/detector circuit and relay. The relay mounts easily on a printed circuit board ("PCB") and operates reliably up to approximately 5 GHz. The RPP limiter/detector circuit functions well up to 4 GHz.
RF instruments such as signal generators, spectrum analyzers, network analyzers, and measuring receivers may be exposed to high power RF signals of 50 W or more if such signals are accidentally applied to the instruments' external signal port. The sensitive internal circuitry of these instruments can be damaged when exposed to such high power signals. To protect the internal circuitry, RPP limiter/detector circuits and relays are used.
Typically, limiting diodes coupled to the external signal port as shunts with a predetermined bias voltage, permitting nominal signal levels in a forward direction but limiting any reverse signal input to the predetermined voltage level. The size of the diodes is limited by the need to match impedances along the signal path, thereby minimizing signal reflection and resulting signal degradation over the operating frequency range. Small diodes have only a small capacitance and therefore affect the overall impedance of the circuit less. Given their small size, these diodes can only protect the internal circuitry from high power signals for a short time. After this short time, the diodes fail, exposing the instrument to the high power RF signal.
To increase the protection of the RF instrument's internal circuitry, a relay is placed in the signal path between the RF output connector and the limiting diodes. Normally, the relay is closed, allowing signals to flow in either direction. In response to the injection of a reverse power signal above a predetermined threshold, the relay is triggered open by an RPP limiter/detector circuit. The open relay saves the limiting diodes and internal circuitry from damage.
The limiting diodes provide an interim time period for protecting the internal circuitry while the reverse power surge is detected by the RPP limiter/detector circuit and the relay switched open. The diodes, the relay and its coupling capacitors, and RPP limiter/detector circuit must together closely match the impedance of the RF instrument to avoid signal reflection and related signal degradation during normal operation.
Known RPP limiter/detector circuits and relays use microstrip transmission lines for both the diodes and transmission line structures. The series inductance created by the lengthy connection between the surface mounted diodes and the ground plane makes the upper frequency limit of these printed circuit microstrip designs approximately 3.5 Ghz. To fabricate RPP limiter/detector circuits that will work at frequencies above 4 Ghz, the path length to ground must be reduced. This has previously been accomplished by using thin circuit material and a microcircuit design with the diode chip bonded onto the microstrip. Such microcircuits are typically more expensive than PCBs and are only used when the desired performance can not be achieved any other way. Even the best known RPP limiter/detector circuits and relays using thick film microcircuits and wire bonds perform relatively poorly above 2 GHz, providing reduced power protection at these higher frequencies, and doing so at relatively high cost.